This invention relates to a joint prosthesis for replacing a diseased or damaged joint between skeletal members, and, more particularly, to a joint prosthesis having a durable, resilient hinging element.
The repair or complete replacement of damaged or diseased skeletal joints is a relatively common surgical procedure. Severe pain and restricted motion of skeletal joints caused by external trauma, rheumatoid arthritis and other conditions have been corrected by surgical procedures in which the skeletal joint is completely removed and replaced by a prosthesis extending into the intramedullary canals of the adjoining skeletal members. Several prior art prostheses for replacement of the fingers, toes, wrist, elbow, ankle and knee joints have been proposed, but problems have been encountered with each design.
One type of prior art prostheses design is exemplified in U.S. Pat. No. 4,352,212 to Greene et al which discloses a joint prosthesis for replacement of the metacarpophalangeal joint. This prosthesis includes one stem portion adapted for insertion into the intramedullary canal of a metacarpal bone and a second stem portion adapted for insertion into the intramedullary canal of a proximal phalanx. The stem portions are pivotally connected by a hinge construction in the form of a sleeve bearing which consists of a metal cylinder pivotal within a socket or cavity formed of rigid plastic or metal. Other prosthesis designs of this general type include a hinge construction for connecting the stem portions in the form of a ball bearing, usually having metal-to-metal bearing surfaces, such as shown in U.S. Pat. No. 4,304,011.
One problem with joint prostheses having hinging elements formed with metal-to-metal or metal-to-plastic bearing elements is that no resiliency or flexibility is provided at the hinging element to cushion and absorb impact loads, shock loads or lateral and compressive loads applied to the joint in everyday use. The stem portions of prostheses such as described in U.S. Pat. Nos. 4,352,212 and 4,304,011, are rigidly secured within the intramedullary canals of the adjoining skeletal members by bone cement, or with bone and tissue ingrowth induced by surface treatment of the stem portions or other known means. It has been found that the rigid metal-to-metal or metal-to-plastic connection of the members forming the hinging elements of such prior art prostheses fail to cushion or reduce the magnitude of impact or shock loads applied to the joint, but instead transmit them directly to the stem portions. These loads often break the engagement between the stem portions of the prosthesis and the bone cement or tissue ingrowth securing them in place within the intramedullary canals of the adjoining skeletal members. The prosthesis thus becomes loosened and dislocated within the skeletal members requiring replacement.
To avoid the rigid hinging connection between the stem portions of joint prostheses so that impact or shock loads are absorbed or cushioned, several designs have been proposed in which the prosthesis is formed almost entirely of a flexible material such as elastomer. This is particularly popular in finger joint prostheses such as shown, for example, in U.S. Pat. Nos. 3,462,765 to Swanson and 3,875,594 to Swanson. The Swanson finger joint is formed entirely of silicone rubber, preferably "Silastic" silicone elastomer which is a registered trademark of Dow Corning Corporation. The Swanson finger joint comprises a pair of silicone rubber stem portions adapted for insertion into the intramedullary canals of adjoining phalanges or a metacarpal bone and phalanx, which are connected by a section of silicone rubber to permit pivoting of the stem portions relative to one another. In the Swanson design, as well as other joint prostheses formed primarily of elastomer, the stem portions are not secured within the intramedullary canals but are movable and loosely fit within the adjoining skeletal members.
The Swanson type prosthesis employs a flexible hinging element between the stem portions within the intramedullary canals and thus provides better cushioning of loads applied to the prosthesis than prior art prostheses having rigid hinging elements. However, a new set of problems are created. Clinical trials have shown that the stem portions of the Swanson joint prosthesis often come completely out of the intramedullary canals of adjoining bones and must be reinserted in another surgical procedure. In addition, the elastomer stem portions and hinging section therebetween are so flexible that dislocation of one skeletal member relative to the adjoining skeletal member cannot be prevented. For example, one phalanx may be permitted to bend along an axis which is offset from the axis of an adjoining phalanx. This condition is common in advanced cases of rheumatoid arthritis. When adjoining phalanges are not aligned with one another, the stem portions are articulated along different axes when the fingers are bent causing a shear force to develop between the stem portions and the elastomer hinging element therebetween. Over a short period of time, this shear force causes the elastomer to tear resulting in complete failure of the prosthesis.
A third type of joint prosthesis has been developed to overcome the limitations of prior art designs, which is shown, for example, in U.S. Pat. No. 4,229,839. This patent discloses a joint prosthesis whose stem portions are adapted to be fixed within the intramedullary canals of the adjoining skeletal members, and whose hinge element provides a resilient connection between the stem portions. The hinging element consists of a rigid pin which is connected to one stem portion by a first body of elastomer and to the other stem portion by a second body of elastomer. The bodies of elastomer help to reduce the magnitude of loads transmitted to the stem portions and, in turn, their connection to the intramedullary canals of the adjoining skeletal members.
One problem with this design is that the bending of one skeletal member relative to the other requires the elastomer bodies to be bent or flexed at their connection to the pin and/or stem portions. In order to accommodate the millions of bending motions which normal skeletal joints undergo, an elastomeric material having a high flex life must be utilized which adds to the expense and difficulty of manufacturing the prosthesis. A second problem with this design is that it is subject to failure where the adjoining skeletal members are articulated or loaded along offset or non-aligning axes. The joint prosthesis may be subjected to off-axis loading or movement where the stem portions are not precisely aligned with the longitudinal axes of the skeletal members in which they are attached, or where the skeletal members themselves do not anatomically align as a result of an arthritic condition or some external trauma. If the stem portion connected to one body of elastomer is not aligned with the stem portion connected to the other body of elastomer, one of the elastomer bodies will be subjected to shear forces when the prosthesis is articulated. Over time, these shear forces can create tears in the elastomer bodies resulting in complete failure of the prosthesis.